A general hydraulic system (or hydraulic circuit) comprises a pressure source including a hydraulic pump for delivering a hydraulic fluid from a tank to the system, an actuator (for example, a hydraulic cylinder or hydraulic motor) for performing a required operation upon receipt of the supply of hydraulic fluid from the pressure source, and a control part (for example, a hydraulic control valve such as a selector valve) for controlling the pressure and flow rate of the hydraulic liquid in the system. Such a hydraulic system can be said as a system for performing an active operation by making good use of fluid energy of the hydraulic liquid.
In contrast, a hydraulic system relating to a human body frequently has no pressure source contrary to the general hydraulic system. As a device for assisting, for example, the movement by a joint such as a knee and an ankle), there is known a device for exerting a resisting force against the movement (for example, bending or flexing) of a human body. For example, Patent Document 1 shows an outfit or orthosis for the aged simulation and more particularly to a torque damper for exerting a resisting force against bending of the knee, Patent Document 2 shows a damper for a thigh prosthesis and more particularly to a unidirectional characteristic rotary damper for exerting a resisting force only when the knee is bent. Patent Document 3 shows a valve device, as a technique for braking the knee, for varying the restricting amount of a valve in accordance with the load of an artificial leg wearer loaded on the knee and generating a braking force (i.e., resisting force) in accordance with the restricting amount.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-147635    Patent Document 2: Japanese Patent Application Laid-Open No. H05-337145    Patent Document 3: Japanese Patent Application Laid-Open No. H08-317944
Such a hydraulic system for a human body is operated to convert a kinetic energy caused by a human body to a fluid energy in order to brake the former kinetic energy (movement of a human body). Therefore, the human body hydraulic system of this type is different from the general hydraulic system and has no pressure source. It performs a passive job. For the sake of convenience of explanation, in the description to follow, a specific application (application for flexibly bending the knee, in other words, for yielding function) in an artificial leg will be described. It should be noted, however, that the present invention relating to a hydraulic system for a human body has various applications such as, an outfit or orthosis for the aged simulation, an artificial leg and a rehabilitation apparatus (for example, an outfit for preventing bending of the knee at the time of rehabilitation).